1. Field of the Invention
The present invention concerns a demodulation method and a demodulator for digital signals with amplitudes having a constant envelope, continuously modulated in phase and/or in frequency.
It concerns, notably, methods for the demodulation of radio-electrical, digital data carrier signals and their uses in systems for the transmission of data by packets or frequency hopping.
In these methods, the items of data to be transmitted are converted into the form of sequences of symbols of constant duration forming the Baud interval of the transmission to modulate, in frequency or phase, a carrier wave with an amplitude having a constant envelope. The value of these methods lies, above all, in the fact, that they lead to lower frequency occupancy rates than those of the transmission systems known as FSK (frequency shift keying) or PSK (phase shift keying) that use rectangular symbols.
2. Description of the Prior Art
An example of a system implementing a method for the modulation of a continuously phase modulated carrier with an amplitude having a constant envelope is known from the French patent application No.82 08043.
According to the modulation system described, the transmission modulator has an encoder, which makes discrete phase jump values correspond to the data or symbols to be transmitted, and a symbol synthesizer which, as a function of the data given by the encoder, prepares a form of continuous phase variation with a minimum slope, expressed as a function of time by a polynomial relationship. The reception demodulator uses synchronous demodulation and operates by correlation to recover the characteristic instants of the modulation of the frequency deviation between the transmission and reception carriers.
This system has the advantage of using a narrow frequency bandwidth with respect to the throughput rate of the data that it transmits and of enabling, for example, transmission of data at 16 K bits/s in 25 KHz wide channels, with an attenuation of 50 dB outside the channel.
However, the demodulator proves to be complicated to make when the data transmission systems have to transmit data in packets or by frequency hopping, notably because of the complexity of the device for the estimation of the symbol clock used. Since this device has one specific function to estimate the the continuous component and one specific function to estimate the symbol clock, conditioned by the estimation of the continuous component, the effects due to the natural noise of the transmission and those due to the frequency deviation of the reception control oscillator with respect to that of the transmission oscillator are not dissociated. The result thereof is that the performance characteristics of the synchronizing circuit keep deteriorating as and when the transmission gets infested with noise.